A non-contact power transmission apparatus that transmits power in a non-contact manner is widespread. The non-contact power transmission apparatus includes a non-contact power supply device that supplies electric power and a non-contact power reception device that receives the electric power supplied from the non-contact power supply device. The non-contact power supply device supplies electric power to the non-contact power reception device in a state of being electromagnetically coupled with the non-contact power reception device by electromagnetic induction or magnetic field resonance. The non-contact power supply device has a charging pad provided with a power transmission coil. The non-contact power supply device supplies electric power to the non-contact power reception device placed on the charging pad by generating a magnetic field from the power transmission coil. The non-contact power reception device includes a rechargeable battery. The non-contact power reception device carries out a charging processing to store the electric power supplied from the non-contact power supply device in the rechargeable battery.
There is also a non-contact power transmission apparatus, which starts power transmission after carrying out an authentication processing with the non-contact power transmission apparatus and the non-contact power reception device. The non-contact power supply device executes the authentication processing with the non-contact power reception device placed on the charging pad by intermittently transmitting the electric power at low power. If the non-contact power supply device determines that the non-contact power reception device placed on the charging pad is a correct apparatus corresponding to the non-contact power supply device, by transmitting the electric power at normal power, the non-contact power supply device supplies the electric power to the non-contact power reception device placed on the charging pad.
In such a non-contact power transmission apparatus, it is assumed that there is a foreign object inserted between the charging pad of the non-contact power supply device and the non-contact power reception device. For example, in a case in which the foreign object is a conductor such as a metal, if the non-contact power supply device supplies the electric power to the non-contact power reception device placed on the charging pad, there is a possibility of generating an eddy current in the conductor and the conductor generates heat. If the foreign object is an electronic component with an antenna such as a non-contact IC card built therein, there is a possibility that the IC in the IC card is damaged by receiving the electric power by the antenna in the IC card. The electric power output from the power transmission coil of the non-contact power supply device is increased according to a load at a secondary side such as the non-contact power reception device and the foreign object. Therefore, the non-contact power supply device determines presence or absence of the foreign object by detecting a current flowing in the power transmission coil or a power transmission circuit. Furthermore, the non-contact power supply device detects the foreign object during the authentication processing and power transmission and stops the power transmission if the foreign object is detected.
The electric power absorbed by the foreign object increases in proportion to the electric power transmitted from the charging pad. For this reason, a detection accuracy of the foreign object increases as the electric power transmitted from the charging pad increases. However, if the non-contact power supply device carries out the authentication processing by intermittently transmitting the electric power at the low power as described above, there is a problem that the detection accuracy of the foreign object during the authentication processing is low.
If the non-contact power supply device determines that the non-contact power reception device placed on the charging pad is a correct device corresponding to the non-contact power supply device, and the non-contact power supply device transmits the electric power at the normal power if no foreign object is detected during the authentication processing, there is a problem that large electric power is supplied to the foreign object if there is a foreign object inserted between the charging pad of the non-contact power supply device and the non-contact power reception device.